1. Field of the Invention
The present invention relates to batteries such as prismatic lithium-ion batteries used for low-rate discharge backup power sources, and battery modules in which a plurality of such batteries are connected to each other. More particularly, the invention relates to batteries and battery modules that use a lightweight and rigid battery case made of plastic and that achieve high reliability and low cost.
2. Description of Related Art
Power sources used for low-rate discharge backup applications, for example, are hermetically sealed in a limited space and are therefore desired to be compact and lightweight, as well as being low in cost. As one type of batteries that can meet such requirements, lithium-ion batteries, which have high energy density and high power density, have been used widely in recent years. Lithium-ion batteries have been used as secondary batteries for various types of mobile devices.
For the lithium-ion batteries, a battery case made of an aluminum laminate sheet or a metallic can, such as an aluminum can and a stainless steel can, have been used as the outer package. A problem with the use of a metallic can is that although it has rigidity against impact from the outside, metal has greater specific gravity than plastic or the like, resulting in a lower mass energy density of the battery. On the other hand, a battery case made of an aluminum laminate sheet can make the battery lightweight. However, it tends to show insufficient strength to resist impact from the outside because the aluminum laminate sheet is soft, and in order to supplement the insufficient strength, it has been necessary to provide additional means such as covering the battery with an outer cover such as a housing having rigidity.
In addition, when lithium-ion batteries employing a battery case made of a metallic can or an aluminum laminate are used to construct a battery module, it has been necessary to use a housing and battery module components that are made of metal or plastic to construct the battery module using the batteries, and the corresponding increase in the parts count has been a problem.
For lead acid batteries, plastic battery cases have been used because they are low in cost. However, the use of plastic battery cases for lithium-ion batteries is particularly problematic because plastic does not provide complete hermeticity. Specifically, since the lithium-ion battery uses an electrolyte solution containing an organic solvent, entry of moisture through the wall of the plastic battery case and leakage of gas generated from the organic solvent of the electrolyte solution through the wall can be significant causes of poor durability of the battery.
In view of this problem, for example, the following techniques have been known in the field of lead acid batteries. Japanese Published Unexamined Patent Application No. 2002-56829 discloses a battery in which a cylindrical metallic container is fitted outside a plastic battery case. Japanese Published Unexamined Patent Application No. 10-106513 discloses a battery structure in which a metal plate is integrally formed in a plastic battery case. These techniques can prevent deformation and fracture of the plastic battery case, thereby improving the strength. Moreover, it is also possible to minimize permeation and evaporation of moisture.
Nevertheless, the techniques as disclosed in Japanese Published Unexamined Patent Application Nos. 2002-56829 and 10-106513 are far from satisfactory in terms of weight reduction and cost reduction because no particular improvements are observed in terms of parts count reduction.